This invention relates to a mechanical seal formed between a stationary portion and a rotary shaft which is rotatable relative to the stationary portion and more particularly to a mechanical seal formed in the shaft sealing portion of the rotary shaft of a compressor, particularly an axial plunger type compressor.
Usually, such a mechanical seal formed between a stationary portion and a rotary shaft consists of a fixed member fixed airtightly to the stationary portion with a small clearance between the fixed member and the rotary shaft and a rotary member joined airtightly to the rotary shaft so as to rotate together with the rotary shaft and is adapted to prevent gas or liquid leakage through the pressurized contact between the rotary member and the fixed member. In a generally known mechanical seal, the fixed member is formed in an annular member (seat ring) to be fixed to said stationary portion. The rotary member is formed in an annular member (follower ring or a rotary ring) to receive the rotary shaft through the center hole thereof and fixed airtightly to the rotary shaft through a sealing member. An annular sealing surface formed so as to protrude on one side of the rotary member is pressed slidably against one side of the fixed member by the resilient force of a spring.
In the mechanical seal of such a constitution, the fixed member (seat ring) and the rotary member (follower ring) are required to be frictionally durable and strong, therefore, it is usual to form the rotary member with sintered carbon or a composite material of a synthetic resin and carbon and to form the fixed member (seat ring) with stellite, a stainless steel or a high silicon cast iron.
In a compressor for an automotive cooler, such as a rotary compressor, an axial plunger type compressor or a reciprocating compressor, a gaseous refrigerant is subjected to compressive actions within a closed circuit, in which the refrigerant is compressed in the compressor, discharged, condensed in a condenser, vaporized and allowed to expand in an evaporator and then, the evaporator gaseous refrigerant is sucked again into the cylinder through the suction port of the compressor. The further detail of this compression-expansion cycle will be described with reference to an axial plunger type compressor hereunder. A lubricating oil is atomized within the compressor housing and is supplied, together with the refrigerant, to portions requiring lubrication, such as a clearance between the piston and the cylinder and the surface of the swash plate to be in contact with the piston shoes. However, excessive lubricating oil supply reduces the cooling efficiency of the cooler due to circulation of the lubricating oil through the closed circuit including the condenser and the evaporator together with the refrigerant, therefore, it is required to separate the lubricating oil from the refrigerant, though very difficult to attain perfectly, or to reduce lubricating oil supply rate. Furthermore, the oil pump is eliminated in some compressors to meet a requirement for reducing the size as well as the weight of the compressor itself to facilitate mounting the compressor on a vehicle.
Since the compressor for an automotive cooler is further driven by an internal combustion engine, it is subjected to unsteady driving speed varying within a wide range of revolving speed from 500 rpm during idling operation to 6500 rpm during high speed running or sudden acceleration and also to variable operating conditions. Besides, reduced lubricating oil supply rate makes extremely difficult for supplying lubricating oil to the sliding portions of the shaft sealing device (mechanical seal) provided for the shaft sealing section of the rotary shaft of the compressor, consequently, the mechanical seal of the shaft sealing section is compelled to be operated under extremely severe operating conditions, in which solid friction between dry sliding surfaces of the sliding portions of the mechanical seal is likely to occur due to insufficient lubrication despite the lubrication with a mixed mist of a lubricating oil and a gaseous refrigerant which are sealed within the refrigerating circuit including the compressor.